Alloys for a heat exchanger tube having an inner protective cladding and brazed disrupter

ABSTRACT

The invention relates to an assembly of two brazing sheets the first one of which consists of a 3xxx alloy clad on one surface with a 1xxx alloy, the second one of which consists of an AA3xxx alloy clad on both surfaces thereof with a 4xxx alloy, which are assembled together by brazing so as to form a disrupted channel for the flow of exhaust gases, in particular for an engine vehicle.

FIELD OF THE INVENTION

The invention relates to the field of brazing sheets for heat exchangertubes made from aluminum alloys, particularly those used for cooling oroptimization of engine efficiency, and heating or air conditioning ofthe passenger compartment.

This is particularly applicable to heat exchanger tubes subject to aseverely corrosive environment such as charged air cooler tubes throughwhich exhaust gas circulation takes place, and usually provided with aturbulator designed to improve heat exchange by increasing the exchangesurface area and disturbing internal fluid circulation.

STATE OF THE ART

Aluminum alloys are used for the fabrication of most automobiles heatexchangers due to their low density which enables a weight saving,particularly compared with copper alloys, while giving good thermalconduction, ease of use and good resistance to corrosion.

All aluminum alloys referred to in the following are designated usingthe designations defined by the Aluminum Association in RegistrationRecord Series that it publishes regularly, unless mentioned otherwise.

Exchangers comprise tubes for the circulation of internal fluid and finsor separators, and possibly a turbulator to increase heat transferbetween the internal fluid and the external fluid, and they arefabricated either by mechanical assembly or by brazing.

In the case of Charged Air Coolers, known to those skilled in the artunder the term CAC, the normal configuration is that the core sheetmaking up the tube (usually called the tube strip) usually made fromAA3xxx aluminum alloy, is coated on its external and internal faces withan alloy called the brazing alloy, usually in the AA4xxx series. Thishas the advantage that it melts at a temperature below the core meltingtemperature and, through application of a thermal brazing cycle, cancreate a bond between two materials to be assembled, in other wordsbrazed, or possibly to bond the tube strip onto itself so as to closethe tube by brazing, the alternative being welding.

This configuration is shown in FIG. 1, the core of the tube strip beingreferenced as mark 2 and its internal and external brazing alloy cladsbeing referenced as mark 1. The separators placed between the differenttube rows are composed of an unclad AA3xxx alloy. Similarly, theturbulators that are inserted into the tubes are also made from anAA3xxx alloy and are not clad. Inserts are brazed on the tubes by the4xxx clad on the outside face of the tube. The turbulators are brazed inthe tubes by the 4xxx clad on the inside face of the tube. The AA3xxxalloys used for the separators and turbulators may or may not beidentical. The AA3xxx alloy used for the tube core is usually aso-called “long-life” alloy, in other words it has good resistance toexternal saline corrosion.

FIG. 2 shows a diagram of such a turbulator tube, the tube beingreferenced as mark 1, the turbulator as mark 3 and the brazing alloyclads as mark 2.

An example is given in application EP 0283937 A1 by Nihon Radiator Co.Ltd.

At the moment there are engine designs in which exhaust gases arereinjected into the charged air cooler to be mixed with clean air andreturned to the inlet, with the final purpose of reducing pollutingemissions of vehicles.

In this configuration, exhaust gases that could condense can generate aparticularly severe corrosive environment in the cooler, characterizedparticularly by a low pH (that can significantly be less than 3).

In the case of a tube configuration according to the state of the art asdescribed above, a significant diffusion of silicon contained in thecladding AA4xxx alloy to the core alloy forming the core of the tubeoccurs during the brazing operation, thereby degrading its resistance tocorrosion.

One solution known to those skilled in the art consists of insertingintermediate cladding during co-rolling, made from an AA1xxx or AA7xxxalloy, between the tube core alloy and its internal coating made fromAA4xxx brazing alloy.

Such a configuration is diagrammatically shown in FIG. 3, the tube corebeing referenced as mark 3, the outside coating made from an AA4xxxalloy being referenced as mark 4, the inside coating also made from anAA4xxx alloy being referenced as mark 1, and intermediate coating madefrom an AA1xxx or AA7xxx alloy being referenced as mark 2.

Such a coating acts by limiting diffusion of silicon from the internalcladding to the tube core during brazing, thus improving its corrosionresistance.

A sacrificial alloy typically in the AA7xxx series, may also be used asthe intermediate cladding alloy.

These “multi-clad” sheets are known to those skilled in the art and aredisclosed particularly in applications JP 2003027166 A by Kobe SteelLtd. Shinko Alcoa, JP 2005224851 A by Shinko Alcoa Kizai KK, WO2006/044500 A2 and WO 2009/142651 A2 by Alcoa Inc, WO 2007/042206 A1 byCorus Aluminium Walzprodukte GmbH, US 2010/0159272 A1 by Novelis, etc.

The use of this type of “multi-clad” sheets in a charged air coolerthrough which exhaust gas passes is disclosed in application WO2008/063855 by Modine Mfg Co.

Another solution disclosed in applications EP 1762810 A1 and US2007/0051503 A1 by Behr America Inc., consists of favoring the creationof a “brown band” known to those skilled in the art, between the AA3xxxalloy core of the turbulator and its cladding composed of an AA4xxxalloy (typically AA4045), during the conventional type brazing insidethe tube. The tube is also typically made from an AA3xxx alloy clad withAA4xxx on both sides. In one highly preferred embodiment, the tube andthe turbulator are actually composed of the same material.

However, although such configurations can slightly improve theresistance of the tube to corrosion, they may be insufficient underparticularly severe load conditions, which is the case for heatexchangers subject to recirculation of exhaust gases, characterizedparticularly by a low pH.

Other solutions consist of application of a surface treatment afterbrazing to improve the internal corrosion resistance of the tube. Thisis the case of the solution disclosed in application FR 2916525 A1 byValeo Systèmes Thermiques, that recommends a coating based on resins.Another example of a surface treatment, in this case an electro-ceramicdeposit, is given in application WO 2010/019664. Application FR 2930023by Valeo Systemes Thermiques mentions the possibility of boehmiting theentire exchanger. Finally, application EP 1906131 A2 by InternationalTruck discloses a solution consisting of applying a metal surfacetreatment based on Ni or Co on the inside face of the tube and on theturbulator.

Finally, other solutions lie in the use of a combination of differentmaterials, particularly aluminum and stainless steel. ApplicationWO2008/095578 by Behr GmbH & Co. thus claims the use of stainless steelfor the disrupter, the tube being made from aluminum while applicationEPI906127 by International Truck discloses an aluminum tube withstainless steel clad on the inside face, still with a stainless steelturbulator.

However, such options are too expensive to provide an industriallysatisfactory product.

PROBLEM THAT ARISES

The invention is aimed at optimizing the choice of materials or brazingsheets made from aluminum alloys designed for production of exchangertubes with brazed turbulator to improve their resistance in a severecorrosive environment like that created by recirculation of automobilevehicle exhaust gases, without increasing the quantity of materials usedor dimensions or the weight, and for which manufacturing conditions(ease of use and cost) starting from brazing sheets are at leastequivalent to solutions according to prior art.

PURPOSE OF THE INVENTION

The purpose of the invention is an assembly of two brazing sheets, thefirst being composed of a AA3xxx clad on one face with on a AA1xxxalloy, and the second sheet being composed of an AA3xxx aluminum alloyclad on both faces with an AA4xxx alloy, the two sheets being assembledto each other by brazing, so as to form a closed channel or tube with aninternal turbulator, inside which exhaust gases flow, particularlyexhaust gases from an automobile vehicle, alone or in association withanother fluid, typically air, the face of the channel exposed to thesegases or to this mix being the face coated with the 1xxx alloy in thefirst sheet forming the channel, the second forming the internalturbulator.

According to one preferred embodiment, the second brazing sheet iscomposed of an aluminum alloy with the following composition (% byweight):

Si: 0.3-1.0 Fe<1.0 Cu: 0.3-1.0 Mn: 0.3-2.0 Mg: 0.3-3.0 Zn<6.0 Ti<0.1Zr<0.3 Cr<0.3 Ni<2.0 Co<2.0 Bi<0.5 Y<0.5, other elements <0.05 each and0.15 total, the remainder being aluminum,

clad on its two faces with an aluminum brazing alloy containing 4 to 15%of silicon and 0.01 to 0.5% of at least one of the elements Ag, Be, Bi,Ce, La, Pb, Pd, Sb, Y or Mischmetall, said two sheets being assembled toeach other by flux-free brazing under a protective atmosphere.

According to an even more preferred embodiment, the second sheet formingthe turbulator is composed of an aluminum alloy with the followingcomposition (% by weight): Si: 0.3-1.0 Fe<0.5 Cu: 0.35-1.0 Mn: 1.0-2.0Mg: 0.35-0.7 Zn<0.2 Ti<0.1 Zr<0.3 Cr<0.3 Ni<1.0 Co<1.0 Bi<0.5 Y<0.5other elements <0.05 each and 0.15 total, the remainder being aluminum,

clad on its two faces with an aluminum brazing alloy containing 4 to 15%of silicon and 0.01 to 0.5% of at least one of the elements Ag, Be, Bi,Ce, La, Pb, Pd, Sb, Y or Mischmetall, said two sheets always beingassembled together by fluxless brazing under a protective atmosphere.

Furthermore, the first brazing sheet forming a channel or a tube, may beprovided with fins or separators on its outside face, themselves madefrom an aluminum alloy core sheet with the following composition (% byweight):

Si: 0.3-1.0 Fe<1.0 Cu: 0.3-1.0 Mn: 0.3-2.0 Mg: 0.3-3.0 Zn<6.0 Ti<0.1Zr<0.3 Cr<0.3 Ni<2.0 Co<2.0 Bi<0.5 Y<0.5 other elements <0.05 each and0.15 total, the remainder being aluminum, clad on its two faces with analuminum brazing alloy containing 4 to 15% of silicon and 0.01 to 0.5%of at least one of the elements Ag, Be, Bi, Ce, La, Pb, Pd, Sb, Y orMischmetall, and assembled on the channel or tube by fluxless brazingunder a protective atmosphere.

Preferably, said fins or separators are made from a core sheet made fromaluminum alloy with composition (% by weight):

Si: 0.3-1.0 Fe<0.5 Cu: 0.35-1.0 Mn: 1.0-2.0 Mg: 0.35-0.7 Zn<0.2 Ti<0.1Zr<0.3 Cr<0.3 Ni<1.0 Co<1.0 Bi<0.5 Y<0.5 other elements <0.05 each and0.15 total, the remainder being aluminum, clad on its two faces with analuminum brazing alloy containing 4 to 15% of silicon and 0.01 to 0.5%of at least one of the elements Ag, Be, Bi, Ce, La, Pb, Pd, Sb, Y orMischmetall, and assembled on the channel or tube by fluxless brazingunder a protective atmosphere.

According to another variant, the brazing sheet forming the channel orthe tube is coated on the outside face of said channel or tube with acladding layer made from an AA4xxx alloy and fitted with fins orseparators on this coat, assembled by brazing.

Furthermore, the channel or tube may be made from a core sheet made froman aluminum alloy with the following composition (% by weight):

Si: 0.3-1.0 Fe<1.0 Cu: 0.3-1.0 Mn: 0.3-2.0 Mg: 0.3-3.0 Zn<6.0 Ti<0.1Zr<0.3 Cr<0.3 Ni<2.0 Co<2.0 Bi<0.5 Y<0.5 other elements <0.05 each and0.15 total, the remainder being aluminum, clad on its outside face withan aluminum brazing alloy containing 4 to 15% of silicon and 0.01 to0.5% of at least one of the elements Ag, Be, Bi, Ce, La, Pb, Pd, Sb, Yor Mischmetall, and fitted with fins or separators on this layerassembled by fluxless brazing under a protective atmosphere.

Also preferably, the channel or tube may be made from a core sheet madefrom an aluminum alloy with the following composition (% by weight):

Si: 0.3-1.0 Fe<0.5 Cu: 0.35-1.0 Mn: 1.0-2.0 Mg: 0.35-0.7 Zn<0.2 Ti<0.1Zr<0.3 Cr<0.3 Ni<1.0 Co<1.0 Bi<0.5 Y<0.5 other elements <0.05 each and0.15 total, the remainder being aluminum, clad on its outside face withan aluminum brazing alloy containing 4 to 15% of silicon and 0.01 to0.5% of at least one of the elements Ag, Be, Bi, Ce, La, Pb, Pd, Sb, Yor Mischmetall, and fitted with fins or separators on this layerassembled by fluxless brazing under a protective atmosphere.

Finally, these brazing sheets may advantageously be used in thecomposition of turbulator tubes in a heat exchanger, particularly of thetype known under the term EGRC (Exhaust Gas Recirculation Cooler) inwhich only the exhaust gases from an automobile vehicle flow, or of thetype known under the term CAC (Charged Air Cooler) in an EGR (ExhaustGas Recirculation) loop, inside which a mix of fresh air and exhaustgases from an automobile vehicle flows.

The invention also applies to this type of heat exchanger tube in whichexhaust gases flow, particularly automobile vehicle exhaust gases, aloneor associated with another fluid, typically air, made from said brazingsheets and also applies to a heat exchanger comprising at least one tubeof this type.

DESCRIPTION OF THE FIGURES

FIG. 1 shows a brazing sheet with three layers, the core sheet beingreferenced as mark 2 and the brazing alloy (also called the claddinglayer), present on each of the faces of the core being referenced asmark 1.

FIG. 2 diagrammatically shows a tube 1 with turbulator 3, a fin orseparator (not shown), outside the tube, that can be assembled on theexternal face of the tube by brazing. Similarly, the turbulator isassembled on the inside face of the tube by brazing. To achieve this,the two faces of the tubes are coated with a material called the brazingor cladding material, usually in the AA4xxx series (mark 2 in thefigure).

FIG. 3 diagrammatically shows the brazing sheet used for the tube inFIG. 2, but in which an intermediate cladding made from an AA1xxx orAA7xxx alloy has been inserted between the tube core alloy and itsinternal coating made from an AA4xxx brazing alloy.

The tube core is referenced as mark 3, the outside coating made from anAA4xxx alloy is referenced as mark 4, the inside coating also made froman AA4xxx alloy is referenced as mark 1, and the intermediate coatingmade from an AA1xxx or AA7xxx alloy is referenced as mark 2.

FIG. 4 diagrammatically shows a tube 1 with a turbulator 4 according tothe invention, said turbulator itself being composed of a brazing sheetclad, and the tube 1 also being made from a brazing sheet composed of acore sheet, typically made from an AA3xxx alloy, coated on the insideface of the tube with a cladding layer 3 made from an AA1xxx alloy andon the outside face with an AA4xxx brazing alloy layer 2, on whichuncoated fins are brazed.

DESCRIPTION OF THE INVENTION

The invention consists of selecting the most appropriate aluminum alloysforming the brazing sheets used to make channels or tubes of a heatexchanger with a turbulator, more particularly adapted to exchangertubes subjected to a severe corrosive environment particularly such astubes through which exhaust gases from an automobile vehiclerecirculate.

The turbulator placed inside the tube will be designed precisely todisturb internal fluid circulation and to increase heat exchange withthe outside medium.

This turbulator typically consists of a sheet corrugated along itslength similar to an exchanger fin or separator, at least within thescope of the invention, this embodiment being well known to thoseskilled in the art, as mentioned in the “State of the art” section.

Still for elements considered within the scope of the invention, boththe tube and the disrupter are made by bending and assembly of brazingsheets, using usual methods known to those skilled in the art.

The turbulator is assembled inside the tube by brazing. The brazingsheet forming the turbulator is made from a core sheet made from anAA3xxx alloy, coated on its two faces with a cladding layer made from aAA4xxx brazing alloy, for this purpose.

The tube itself is composed of another brazing sheet consisting of acore sheet made from an AA3xxx alloy, and the invention consistsparticularly of forming a lining by cladding on the tube inside face, byselecting an AA1xx alloy for this purpose.

According to the invention, this tube may be coated on its outside facewith a cladding layer made from an AA4xxx alloy to enable brazing ofuncoated fins, also called separators, on said outside face of the tubeor possibly brazing of the tube on itself.

The different alloy clads making up the brazing sheets may typically bemade by co-rolling, a frequently used method known to those skilled inthe art.

Patent EP 1687456 B1 issued by the applicant discloses a brazing sheetcomposition consisting of a core sheet made from an AA3xxx alloy coatedon one or both faces with an AA4xxx brazing alloy making fluxlessbrazing possible under a controlled atmosphere (also called a protectiveatmosphere), typically of nitrogen and/or argon, in a furnace used in astandard manner for use of the Nocolok® process.

This brazing sheet is composed of a core sheet made from an aluminumalloy with the following composition (% by weight):

Si: 0.3-1.0 Fe<1.0 Cu: 0.3-1.0 Mn: 0.3-2.0 Mg: 0.3-3.0 Zn<6.0 Ti<0.1Zr<0.3 Cr<0.3 Ni<2.0 Co<2.0 Bi<0.5 Y<0.5 other elements <0.05 each and0.15 total, the remainder being aluminum,

coated on at least one face with an aluminum brazing alloy containing 4to 15% of silicon and 0.01 to 0.5% of at least one of the elements Ag,Be, Bi, Ce, La, Pb, Pd, Sb, Y or Mischmetall.

An even more preferred composition for the core alloy is as follows (%by weight): Si: 0.3-1.0 Fe<0.5 Cu: 0.35-1.0 Mn: 1.0-2.0 Mg: 0.35-0.7Zn<0.2 Ti<0.1 Zr<0.3 Cr<0.3 Ni<1.0 Co<1.0 Bi<0.5 Y<0.5 other elements<0.05 each and 0.15 total, the remainder being aluminum, and thecoating(s) remaining unchanged.

According to two preferred embodiments of the invention, this first typeof sheet and more advantageously the second type coated on both faces,is selected for making the turbulator, that can thus be assembled byfluxless brazing under a protective atmosphere inside the tube, thusresulting in a considerable cost saving and preventing any risk relatedto the possible entrainment of residual flux into the circuit.

Similarly these two alloy compositions in the same coating configurationon two faces, can advantageously be used for making fins or separatorsand their assembly by fluxless brazing on the uncoated, outside face ofthe tube or channel, in other words directly on the core AA3xxx alloy ofthe tube, in contact with the AA4xxx alloy of the fins according to theabove-mentioned preferred compositions.

Obviously, still according to the invention, it would also be possibleto coat the outside of the tube or channel with a cladding layer ofAA4xxx brazing alloy, so that clad or unclad standard fins or separatorscould be assembled to it by brazing.

Furthermore, the two types of sheets with the above-mentionedcompositions may advantageously be used for making the tube or channelitself, always coated on the inside face with a cladding layer made froman AA1xxx alloy and on its outside face with an AA4xxx brazing alloyenabling fluxless brazing of standard fins under a protectiveatmosphere.

Obviously, the invention is very useful particularly for heat exchangerscomprising this type of tube and more particularly heat exchangers knownto those skilled in the art under the name EGRC (Exhaust GasRecirculation Cooler), in which only the exhaust gases from anautomobile vehicle flow or of the type known as Charged Air Cooler (CAC)in an EGR (Exhaust Gas Recirculation) loop inside which a mix of freshair and automobile vehicle exhaust gas mix flows, in both casesparticularly corrosive media.

Details of the invention will be better understood after reading thefollowing examples that are in no way limitative.

EXAMPLES

Several sheets of 3916 core alloy and AA4045 brazing alloy were castwith AA1050 alloy sheets and one sheet of each alloy 3920 and 4945.

The 3916 alloy had the following composition (% by weight):

Si: 0.18 Fe: 0.15 Cu: 0.65 Mn: 1.35 Ti: 0.08 other elements <0.05 eachand 0.15 total, remainder being aluminum,

The 3920 alloy had the following composition (% by weight):

Si: 0.5 Fe: 0.15 Cu: 0.5 Mn: 1.65 Mg: 0.5 Ti: 0.08 other elements <0.05each and 0.15 total, remainder being aluminum.

The 4945 alloy is an AA4045 alloy that also contains 0.15% of Bi.

Assemblies were made from these sheets to obtain the claddingpercentages (% total thickness) indicated in table 1 below, at the endof the transformation.

These assemblies were hot and then cold rolled to produce 0.40 mm thickclad strips for the tubes and 0.20 mm thick clad strips for theturbulators. A 2 h restoration treatment at 280° C. was then applied tothese strips after a temperature rise at a rate of 45° C./h.

Tube mockups were fitted with turbulators brazed using the Nocolok®process for references 1 to 3 in table 1 and fluxless brazed forreference 4, with a temperature increase phase up to 600° C. at a rateof about 40° C./min, constant temperature for 2 min at 600° C. and thena temperature reduction at a rate of about 50° C./min, all undernitrogen with a flow of 8 l/min.

The tested configurations are shown in table 1 below.

Configuration 1 applies to a simple tube made from a 3916 core alloycoated on its two faces with an AA4045 brazing alloy and a turbulatormade from a AA3003 alloy.

Configuration 2 is identical except for an additional and intermediatelayer of AA1050 alloy between the core and the AA4045 internal brazingalloy.

Configuration 3 according to the invention uses a tube made from a 3916core alloy coated on the outside with an AA4045 brazing alloy and on theinside with an AA1050 brazing alloy and a turbulator made from a 3916core alloy coated on both sides with an AA4045 brazing alloy.

Brazing for these three cases was done using the Nocolok® process withflux.

Configuration 4 according to the invention uses a tube made from a 3916core alloy coated on the outside with an AA4045 brazing alloy and on theinside with an AA1050 alloy and a turbulator made from an AA3920 corealloy coated on both faces with a 4945 brazing alloy (containing 0.15%of Bi) in accordance with the above-mentioned preferred compositions.

In this case, brazing was done fluxless under a protective atmosphere inaccordance with the patent deposited by the above-mentioned applicant.

TABLE 1 Configuration of mockups tested in corrosion Tube TurbulatorRef. Material Material 1 4045 7.5%/3916/4045 7.5% 3003 2 40457.5%/3916/1050 15%/ 3003 4045 7.5% 3 4045 10%/3916/1050 10% 404510%/3916/4045 10% 4 4045 10%/3916/1050 10% 4945 10%/3920/4945 10%

The resistance of tubes to corrosion was tested by means of a corrosiontest alternating immersion and emersion in order to reproduce theconditions applied to CAC (Charged Air Cooler) type exchangers in an EGR(Exhaust Gas Recirculation) low pressure loop. The detailed testconditions used are described in tables 2 and 3 below.

In practice, the test cycle consists of two repetitions of steps 1, 2and 3 and then three repetitions of steps 4, 5 and 6 and the completecycle is repeated four thousand times.

TABLE 2 Corrosion test parameters Repetition Step Liquid DurationTemperature Position X 4000 X2 1 L1 15 s ambient immersion 2 Air 45 sambient emersion 3 Air 15 min 170° C. emersion X3 4 L1 15 s ambientimmersion 5 Air 45 s ambient emersion 6 Air 15 min ambient emersion

TABLE 3 Composition of L1 used for the corrosion test (in ppm) SulfateNitrate Acetic Formic Propanoic Chloride ions ions acid acid acid ionspH 320 52 590 3167 474 20 2.5

Sulfate ions were introduced in the form of sulfuric acid, nitrate ionsin the form of acetic acid and chloride ions in the form of hydrochloricacid.

For each tested configuration, the resistance to corrosion was evaluatedby metallographic observation on a cross-section of the tube and of theturbulator.

The results obtained from characterizations made on samples after 300 hare summarized in table 4 below.

TABLE 4 Characterization of the corrosion test after 300 hTube-turbulator Ref. Tube Turbulator joints 1 Very severe, Corrosion byLittle corrosion. intergranular local pitting. No Some separatedcorrosion. intergranular joints Some leaks corrosion 2 Severe, Corrosionby Little corrosion. intergranular local pitting. No Some separatedcorrosion. intergranular joints Some leaks corrosion 3 Very little andIntergranular Little corrosion. very shallow corrosion No separatedcorrosion. No joints intergranular corrosion 4 Very little andIntergranular Little corrosion. very shallow corrosion No separatedcorrosion. No joints intergranular corrosion

The tubes in configurations 1 and 2 have severe corrosion that canresult in perforations and a significant deterioration of some brazingjoints between the tube and the turbulator. The unclad turbulatorsassociated with this type of tube are only slightly corroded.

On the other hand, configurations 3 and 4 according to the inventionshow very little tube corrosion, often limited to cladding. Corrosion ofbrazing joints is also limited. Turbulators associated with this type oftube are significantly corroded. However this situation is considered tobe significantly better.

A single perforation on a tube causes a leak in the complete heatexchanger circuit. A leak on a turbulator causes a reduction in the heatexchange quality of the exchanger.

Therefore it is easy to understand that it is more important to maintainthe integrity of the tube than the integrity of the turbulator.

1-13. (canceled)
 14. An assembly of two brazing sheets, comprising: afirst sheet comprising an AA3xxx alloy clad on one face with an AA1xxxalloy, a second sheet comprising an AA3xxx aluminum alloy clad on bothfaces thereof with an AA4xxx alloy, wherein said two sheets areassembled to each other by brazing, so as to form a closed channeland/or tube with an internal turbulator, inside which exhaust gas canflow, optionally exhaust gas from an automobile vehicle, alone and/or inassociation with another fluid, optionally air, and further wherein aface of the channel exposed to said gas is said face coated with saidAA1xxx alloy in the first sheet forming the channel, and wherein thesecond sheet forms the internal turbulator.
 15. The assembly of twobrazing sheets according to claim 14, wherein said second brazing sheetcomprises an aluminum alloy with the following composition (% byweight): Si: 0.3-1.0 Fe<1.0 Cu: 0.3-1.0 Mn: 0.3-2.0 Mg: 0.3-3.0 Zn<6.0Ti<0.1 Zr<0.3 Cr<0.3 Ni<2.0 Co<2.0 Bi<0.5 Y<0.5, other elements <0.05each and 0.15 total, remainder being aluminum, And further wherein saidsecond brazing sheet is clad on two faces thereof with an aluminumbrazing alloy comprising from 4% to 15% of silicon and from 0.01% to0.5% of at least one of Ag, Be, Bi, Ce, La, Pb, Pd, Sb, Y and/orMischmetall, said two sheets being assembled to each other by fluxlessbrazing under a protective atmosphere.
 16. The assembly of two brazingsheets according to claim 15, wherein said second sheet comprises analuminum alloy with the following composition (% by weight): Si: 0.3-1.0Fe<0.5 Cu: 0.35-1.0 Mn: 1.0-2.0 Mg: 0.35-0.7 Zn<0.2 Ti<0.1 Zr<0.3 Cr<0.3Ni<1.0 Co<1.0 Bi<0.5 Y<0.5 other elements <0.05 each and 0.15 total,remainder being aluminum, wherein said second sheet is clad on two facesthereof with an aluminum brazing alloy comprising from 4% to 15% ofsilicon and from 0.01% to 0.5% of at least one of Ag, Be, Bi, Ce, La,Pb, Pd, Sb, Y and/or Mischmetall, said two sheets being assembledtogether by fluxless brazing under a protective atmosphere.
 17. Theassembly of two brazing sheets according to claim 14, wherein saidassembly is provided with at least one fin and/or separator on anoutside face of the first sheet forming the channel, said fin and/orseparator comprising an aluminum alloy core sheet with the followingcomposition (% by weight): Si: 0.3-1.0 Fe<1.0 Cu: 0.3-1.0 Mn: 0.3-2.0Mg: 0.3-3.0 Zn<6.0 Ti<0.1 Zr<0.3 Cr<0.3 Ni<2.0 Co<2.0 Bi<0.5 Y<0.5 otherelements <0.05 each and 0.15 total, remainder being aluminum, clad ontwo faces thereof with an aluminum brazing alloy comprising from 4% to15% of silicon and from 0.01% to 0.5% of at least one of Ag, Be, Bi, Ce,La, Pb, Pd, Sb, Y and/or Mischmetall, and said fin and/or separatorbeing assembled on the channel by fluxless brazing under a protectiveatmosphere.
 18. The assembly of two brazing sheets according to claim14, wherein said assembly is provided with at least one fin and/orseparator on an outside face of the first sheet forming the channel,said fin and/or separator comprising an aluminum alloy core sheet withthe following composition (% by weight): Si: 0.3-1.0 Fe<0.5 Cu: 0.35-1.0Mn: 1.0-2.0 Mg: 0.35-0.7 Zn<0.2 Ti<0.1 Zr<0.3 Cr<0.3 Ni<1.0 Co<1.0Bi<0.5 Y<0.5 other elements <0.05 each and 0.15 total, remainder beingaluminum, clad on two faces thereof with an aluminum brazing alloycomprising from 4% to 15% of silicon and from 0.01% to 0.5% of at leastone of Ag, Be, Bi, Ce, La, Pb, Pd, Sb, Y and/or Mischmetall, andassembled on the channel by fluxless brazing under a protectiveatmosphere.
 19. The assembly of two brazing sheets according to claim14, wherein the channel is coated on an outside face thereof with acladding layer made from an AA4xxx alloy and fitted with at least onefin and/ or separator on said cladding layer, and being assembled bybrazing.
 20. The assembly of two brazing sheets according to claim 14,wherein the channel is made from a core sheet comprising an aluminumalloy with the following composition (% by weight): Si: 0.3-1.0 Fe<1.0Cu: 0.3-1.0 Mn: 0.3-2.0 Mg: 0.3-3.0 Zn<6.0 Ti<0.1 Zr<0.3 Cr<0.3 Ni<2.0Co<2.0 Bi<0.5 Y<0.5 other elements <0.05 each and 0.15 total, remainderbeing aluminum, clad on an outside face thereof with an aluminum brazingalloy containing from 4% to 15% of silicon and from 0.01% to 0.5% of atleast one of Ag, Be, Bi, Ce, La, Pb, Pd, Sb, Y and/or Mischmetall, andsaid channel being fitted with at least one fin and/or separator on theclad, and being assembled by fluxless brazing under a protectiveatmosphere.
 21. The assembly of two brazing sheets according to claim14, wherein the channel is made from a core sheet comprising an aluminumalloy with the following composition (% by weight): Si: 0.3-1.0 Fe<0.5Cu: 0.35-1.0 Mn: 1.0-2.0 Mg: 0.35-0.7 Zn<0.2 Ti<0.1 Zr<0.3 Cr<0.3 Ni<1.0Co<1.0 Bi<0.5 Y<0.5 other elements <0.05 each and 0.15 total, remainderbeing aluminum, clad on an outside face thereof with an aluminum brazingalloy comprising from 4% to 15% of silicon and from 0.01% to 0.5% of atleast one of Ag, Be, Bi, Ce, La, Pb, Pd, Sb, Y and/or Mischmetall, andsaid channel being fitted with at least one fin and/or separator on saidclad and being assembled by fluxless brazing under a protectiveatmosphere.
 22. The assembly of two brazing sheets according to claim14, wherein said assembly forms part of a heat exchanger.
 23. Theassembly of two brazing sheets according to claim 22, wherein saidassembly forms at least part of a heat exchanger known under the termEGRC (Exhaust Gas Recirculation Cooler) in which only exhaust gas froman automobile vehicle flows.
 24. The assembly of two brazing sheetsaccording to claim 22, wherein said assembly forms at least part of aheat exchanger known under the term CAC (Charged Air Cooler) in an EGR(Exhaust Gas Recirculation) loop, inside which a mix of fresh air andexhaust gas from an automobile vehicle flows.
 25. A heat exchanger tubein which exhaust gas flows, optionally automobile vehicle exhaust gas,alone or associated with another fluid, optionally air, comprising anassembly of brazing sheets according to claim
 14. 26. A heat exchangercomprising at least one tube according to claim 25.